The present invention relates to an apparatus for gasifying or combusting solid, carbonaceous material in a circulating fluidized bed reactor. The circulating fluidized bed reactor comprises a separator which is disposed after the reaction chamber and which separates circulating bed material from gas. The separator for circulating material is provided with a gas outlet for discharging gas from said separator and with a duct for returning separated particles preferably to the lower part of the reaction chamber. A separator for fine particulates is also disposed in the gas flow from the separator.
In a circulating fluidized bed reactor, where the flow rate of gas is maintained at such a high level that a considerable portion of solid particles is discharged with gas from the reaction chamber and, after separation of particles, the major part of the solid particles is returned to the fluidized bed, the gasification or combustion of solid carbonaceous material has been recognized to have several advantages over conventional gasification or combustion methods.
Several different methods have been applied in the gasification of solid, carbonaceous fuel, the most important of them being gasifiers based on the fluidized bed concept as described above. The problem with all gasifiers, including (although to a lesser extent) fluidized bed gasifiers, is how to achieve a very high carbon conversion. This problem becomes more acute when fuels with low reactivity, such as coal, are to be gasified. It is also difficult to achieve a high carbon conversion with fuels having a small particle size, such as milled peat.
Poor carbon conversion is principally the result of the comparatively low reaction temperature of fluidized bed gasifiers, which is restricted by the melting temperature of the fuel ashes. Carbon conversion can be significantly improved by increasing the reaction time of the gasification, i.e., by returning the escaped, unreacted fuel to the reactor.
In a circulating fluidized bed gasifier or boiler, the rate of flow of the upwardly directed gas is so high that a substantial amount of solid bed material, entrained with product or flue gases, passes out of the reactor. Most of such outflowing bed material is separated from the gas by separators and returned to the reactor. The finest fraction, however, is discharged with the gas. Circulating material in the reactor comprises ashes, coke and other solid material, such as limestone, possibly introduced in the gasifier, which induces desired reactions such as sulfur capture.
However, separators such as cyclones, which are normally used, have a restricted capacity for separating small particles. Normally, hot cyclones can separate only particles up to the size of 50-100 microns, and finer fractions tend to escape with the gases. Since the unreacted fuel discharged from the reactor with the gas is mainly coke, from which the volatile (reactive) parts have already been discharged, it would, when returned to the reactor, require a longer retention time than the actual "fresh" fuel. However, because the grain size of the returned coke is very small, the returned fine fraction is immediately discharged again from the reaction chamber, and thus the reaction time remains too short and the carbon conversion undesirable low.
The grain size of the coke gradually becomes less during the process, thereby increasing the emission of particulate material from the cyclone, which results in a low carbon conversion.
Even though small coke particles can be separated from the gases with new ceramic filters, additional problems arise. Solid fuels always contain ashes which have to be removed from the system when pure gas is produced. When aiming at a carbon conversion as high as possible, ashes have to be removed so as to avoid discharging large amounts of unreacted carbon with the ashes. The particle size of the ashes, however, always varies within a wide range and fine ashes tend to fly out of the reactor with the fine coke residue.
In order to achieve a high carbon conversion, the following diverse criteria must be reconciled: (1) separation of fine particulates from the gases and return of such to the reactor must be possible, and (2) the carbon contained in the returned particulates has to be made to react, and the ashes have to be separated from the system.
Until now, attempts to reconcile these criteria have been unsuccessful.
It is also common in boiler plants, at fluidizing bed combustion, that unburned coal is easily entrained with the fly ash, especially if poorly reactive fuel is employed or if the boiler plant is under a low load or under an extremely heavy load. Fly ash may contain over 10% coal, sometimes even 20%, which lowers the efficiency of the boiler. It is known that returning the fly ash to the combustion chamber gives a lower carbon content in the fly ash, thus improving the efficiency of the boiler.
Fly ash itself is a problematic produce, however. For example, in the U.S.A., only 20% of the total amount of fly ash can be utilized in the building industry and construction of roads. Final storing causes problems to the power plants. Fly ash is a low density material which means that the residual fly ash requires quite a large storage area. This constitutes a problem in densely populated areas. Furthermore, one has to pay attention to storing of the ashes in such a manner that they do not come into contact with groundwater. Ammonia has recently been introduced into the purification of flue gases, and this has added to the fly ash problem. The fly ash treated with ammonia cannot be used in the concrete industry.
The combustion temperatures in the fluidized bed boilers are substantially lower than, for example, in pulverized combustors and the ash properties are quite different. Ashes produced by combustion at lower temperatures are not stable, but depending on the conditions, there may be gaseous, liquid or dusty emissions.
U.S. Pat. No. 4,315,758 discloses a method and apparatus for solving the problem with the fines recycling. According to this method, the finest particulates separated from the gas are conducted back to the lower part of the reactor while oxygen containing gas is introduced into the same place in the reactor, thereby forming a high temperature zone in which the recovered fine particulates agglomerate with the particles in the fluidized bed. This method introduces an improvement in the so-called "U-gas Process" method.
British Patent No. =GB 2,065,162 discloses a method and apparatus for feeding the fine material separated from gas to the upper part of the fluidized bed in which the fine particulates agglomerate with particles of the fluidized bed when oxygen containing gas is conducted to the same place in the reactor.
The problem with both of these methods is process control. Both methods aim at agglomeration of the separated fine material to the fluidized bed (featuring excellent heat and material transfer properties). It is of major importance that the main process itself can operate at an optimal temperature, and it is easily disturbed when the temperature needed for the agglomeration is not the same as that needed for the main process. Due to the good heat transfer that occurs in the fluidized bed, the temperatures tend to become balanced, which causes new problems. Gas different from the oxygen containing gas used in the actual gasification is needed because of the excess heat. Additionally, because the size of particles contained in the fluidized bed varies considerably, it is difficult to control the agglomeration in the reactor so that production of ash agglomerates of too large a size could be prevented. Ashes stick to large as well as small bed particles and ash agglomerates of too large a size are easily formed, which impede or prevent ash removal, and the gasifying process has consequently to be interrupted. Furthermore, agglomeration in the reactor itself causes local overheating, which in turn leads to abrasion of brickwork.
U.S. Pat. No. 3,847,566 discloses one solution in which high carbon conversion is sought by burning the fine material escaping from the gasifier in a separate combustion device. Coarser, carbonaceous material taken from the fluidized bed is heated with the heat released from combustion. This carbonaceous material is returned to the fluidized bed after the heating. This is how the heat required for the gasification is generated. The gases, flue gas and product gas, released from the combustion and gasification have to be removed from the system in two separate processes both including a separate gas purification system. As can be seen, the arrangement of this method requires quite complicated constructions and results in poorly controlled processes.
The problem with the above-mentioned methods resides in the difficult process conditions where agglomeration conditions have to be controlled. This calls for expensive materials and cooled constructions.
According to the invention, an apparatus for gasification or combustion, by means of which the highest possible carbon conversion is attained without the above-mentioned drawbacks in the process control and without complicated and expensive constructions, is provided. According to the invention, it is also possible to separate the finest carbonaceous particulates from the product or flue gas and return them to the reactor in such a form that the carbon contained in the particulates can be exploited and the ash be separated.
According to the invention, in a circulating fluidized bed reactor, agglomerating means are provided comprising an agglomerating and fluidizing chamber disposed in connection with the return duct for particles. The agglomerating chamber is in communication with a return duct for circulating particles discharged from the separator and with the lower part of the return duct, through which circulating particles are returned to the lower part of the fluidized bed reactor. The bottom of the chamber is provided with means (such as fluidizing gas nozzles) for feeding fluidizing gas to maintain a bubbling fluidized bed in the chamber. The bed material is comprised of circulating particles. The fluidizing gas can also be introduced into the reactor, for example, through a porous bottom plate.
The upper part of the agglomerating chamber is provided with a burner for particulates for heating and for at least partially combusting fine particulates. Fine particulates from the separator therefor are conducted through the burner to the free space above the fluidized bed in the chamber. The burner for particulates comprises two conduits or nozzles, one for oxygen containing gas, the other for fine particulates or for a mixture of particulates and gas. The burner for particulates is disposed in the upper part of the agglomerating chamber so as to form a flame, produced in the combustion of particulates, substantially in the space above the fluidized bed. Therefore, the nozzle for particulates and the nozzle for gas are preferably so disposed as to make the point of the flame of combusting particulates penetrate the bubbling fluidized bed.
In the apparatus according to the invention, fine particulates separated in the gas purification stage are agglomerated with the circulating bed material at a raised temperature before the solid particles are returned to the reaction chamber. Thus, particles are separated from the gas at least in two stages. In the first stage, mainly coarser particles are separated, the major part of which is returned to the reactor as circulating material. In the second stage, mainly finer carbonaceous particulates are separated, at least a portion of which, agglomerated and mixed with the circulating material, is returned to the fluidized bed reactor at a raised temperature.
The temperature of the separated fine particulates is raised to over 1000.degree. C., preferably to 1000.degree.-1300.degree. C., by conducting oxygen containing gas into the flow of particulates, whereupon at least part of the fine particulates form or become sticky particles which are caused to agglomerate with the circulating particles before they are returned to the reactor chamber. Preferably, agglomerated particles are caused to mix evenly with the circulating particles before they are returned to the reactor.
In such processes where the higher the temperature for purification of the gas the better, fine particulates can also be separated from the product gas by employing several consecutively connected cyclones, cyclone radiators or high-heat filters or other equivalent means which are also capable of separating fine particulates.
On the other hand, for example, connected with a combined power plant, it is advantageous to use the hot product gas for superheating steam and not to separate the fine particulates from the product gas until the gas has cooled to a lower temperature, such as 850.degree. C. In this case, the purification of the gas is also easier to accomplish. At a lower temperature, the gas does not include to a harmful extent fine fumes which are difficult to separate and which easily clog, for example, pores of ceramic filters. Furthermore, hot fumes are extremely aggressive chemically and impose great demands on materials. The method according to the present invention is, therefore, most suitable for combined power plant applications because the carbon conversion of the fuel is high, the product gas is pure and well applicable to gas turbines and, furthermore, the overall heat economy is improved by superheating the steam.
Agglomeration increases the particle size of fine particulates to such an extent that the retention time of the particulates becomes longer in the reactor and the carbon conversion is improved. If the particle size of the returned particulates is increased sufficiently, the ash particles can be removed from the reactor at an optimal stage, whereby the carbon contained in ash particles has reacted almost completely.
By agglomerating the particulates outside the actual fluidized bed reactor, where the coarsest circulating particles are considerably smaller in size than the coarsest fluidizing particles in the reactor itself, formation of particles of too large a size is avoided, which particles might be discharged from the reactor along with the ashes thereby leaving the carbon insufficient time to react completely.
Gasification in a circulating fluidized bed reactor is in some ways different from gasification in a conventional bubbling fluidized bed reactor. In a circulating fluidized bed reactor, the upwardly directed flow rate is so high, typically 2-10 m/s, that a large amount of solid bed material is entrained with the gases to the upper part of the reactor, and some passes out of the reactor, where it is returned after the gas separation. In such a reactor, the important reactions between the gases and solid material are effected over the entire area of the reactor while the suspension density is even in the upper part of the reactor 0.5-30 kg/kg of gas, most commonly 2-10 kg/kg of gas.
In a bubbling fluidized bed, where the flow rate of the gas is typically 0.4-2 m/s and the suspension densities in the upper part of the reactor about 10 to 100 times lower than in the circulating fluidized bed reactor, the gas/solid material reactions are mainly effected in the lower part of the reactor, i.e., in the bed.
The invention provides, for example, the following advantages:
A high degree of carbon conversion is achieved. PA0 Agglomeration of fine carbon can be effected in a controlled manner not disturbing the process conditions in the gasifier or boiler. PA0 With a circulating fluidized bed concept, the cross section of the reactor can be clearly smaller than with a so-called bubbling fluidized bed reactor. PA0 Thanks to the smaller cross section and better mixing conditions, there is an essential decrease in the need for fuel feed and ash removal devices in comparison with the so-called bubbling bed. PA0 Capture of sulfur contained in the fuel with inexpensive lime can be effected in the process. PA0 Reactions between solids and gases take place over the entire area of the reactor section and separator. PA0 The equipment described above does not require expensive special materials. PA0 As the various stages of the process are performed in various devices, the process control can be carried out optimally with regard to the total result. PA0 Inert ashes are received; and PA0 Problems with storing fly ash are reduced.